1. Field of the Invention
The invention relates to a light-reproduction medium such as an optical disc and an optical card to be used to reproduce data stored therein by projecting lights thereto and analyzing lights reflected therefrom. The invention is also related to a method of reproducing data stored in a light-reproduction medium, and a reproduction apparatus for reproducing data stored in a light-reproduction medium. In particular, the invention relates to a light-reproduction medium capable of accomplishing highly densified recording.
2. Description of the Related Art
Various attempts have been made to highly densify a light-reproduction medium such as CD-ROM which is an optical disc used only for data regeneration. An example of CD-ROM is illustrated in FIGS. 1A and 1B which a plan view of a part thereof and a cross-sectional view taken along the line B1--B1 in FIG. 1A. As illustrated, CD-ROM comprises a substrate 1 formed on a surface thereof with a plurality of recessed portions each of which acts as a recording pit 2, and a reflection film 7 covering the surface of the substrate 1. In operation, lights are projected to the surface of the substrate 1 with one of an optical head or the CD-ROM being moved in a direction indicated with an arrow Y. and lights reflected from the substrate 1 are detected. Then, the reflected lights are detected to thereby identify recessed or raised portions of the surface of the substrate 1 of which the recording pit 2 is constituted. The thus detected light intensity is converted into signals, and thus data stored in the CD-ROM are reproduced. Hereinbelow, though not explained each time, an optical disc such as CD-ROM is to be moved relative to an optical head between a top and a bottom of a figure, namely in a direction indicated with the arrow Y in FIG. 1A, for instance.
For fabrication of such CD-ROM as above mentioned, there is used a basic pattern board called a stamper which is formed on a surface thereof with recessed and raised portions acting as recording pits storing data therein. By injection mold technology, the recessed and raised portions are copied on a polycarbonate substrate, and then a reflection film composed of metal such as aluminum is deposited on the polycarbonate substrate. The stamper is fabricated with exposure technology called mastering, and hence, the memory density of CD-ROM is determined in dependence on how fine recording pits can be fabricated with the mastering.
In order to meet the demand of highly densified light-reproduction medium in these days, it has been proposed as one solution to fabricate a recording pit in smaller size. To this end, it is necessary to fabricate the recessed and raised portions of the stamper in smaller size, which in turn requires a smaller-sized mastering. However, there is a limitation in reduction of size of a mastering, and hence it is almost impossible to fabricate a higher density light-reproduction medium. Thus, a new technique for producing a higher density light-reproduction medium is necessary to be developed in place of the mastering.
For instance, Japanese Unexamined Patent Publication No. 47-26104, which is based on Dutch application No. 7103233 filed on Mar. 11, 1971, has suggested a circular disc shaped data carrier which contains aural and video signals encoded to optical signs or which contains the aural and video signals in swirl arrangement, and also has suggested a reader for the data carrier.